Various methods such as roll coating, gravure coating and extrusion coating are used to apply a coating solution, a liquid coating composition, to beltlike materials such as photographic films, photographic papers and magnetic recording mediums. In particular, it is most common to use extrusion coating since it can form uniform coatings.
In the extrusion coating, the shape of a die edge is important in order to stably carry out uniform coating, as disclosed in U.S. Pat. Nos. 5,042,422 and No. 5,097,792.
In such extrusion coating, however, the surface of the beltlike material continuously running may come into touch with the edges of a coater die. For this reason the die edges can not avoid being worn because of the presence of hard fine particles in coating solutions, and a slight deformation due to wear may cause streak defects, resulting in an extreme lowering of coatability. For example, in the case of coating solutions for magnetic recording mediums, it is an important subject to prevent wear of the die edges, since coating solutions often contain fine hard particles, e.g., abrasives such as aluminum oxide and chromium oxide or ferromagnetic powder, and besides there is a demand for making coating speed as higher as possible (100 m/min to 1,000 m/min or more) for the purpose of improving productivity. In the case when coating solutions contain fine hard particles, the slits of the coater die, through which coating solutions pass, may broaden because of their wear to cause a change in extrusion rate of the coating solutions. Thus, it is another subject to prevent wear of the slits.
In addition, in order to stably carry out coating that can be uniform in the width direction, the coater die used in extrusion coating is required to achieve a dimensional precision that is uniform in its width direction. Its precision must be of a micrometric or higher order. Hence, the coater die is usually worked by precision grinding or polishing. During the grinding or polishing, fine particles of the abrasive wheel or abrasive grains used can be included in the working surface of the die. Such fine particles of the abrasive wheel or abrasive grains, included in the edge surface, may come off as s result of peripheral abrasion to cause a deformation of edges and cause troubles such as streak defects, resulting in an extreme lowering of coating performance.
Inclusion of the fine particles of the abrasive wheel or abrasive grains may also cause a corrosion from that portions.
Moreover, particularly in the case of mediums for magnetic recording, there is an increasing demand for smaller film thickness. When film thickness is made smaller, the use of a coater die for a long period of time brings about a problem of low electrical characteristics caused by coating uneveness, coating surface roughness ascribable to the coater die, etc.
In order to prevent wear of die edges or slits, it has been hitherto attempted to increase the strength of die edges by forming the edges of the coater die with a hard metal as disclosed in U.S. Pat. No. 4,424,762, or by forming the coater die itself with a ceramic material as disclosed in .European Patent Publication No. 361,167.
Highly wear resistant materials such as hard metals or ceramic materials, however, bring about an increase in cost of materials and at the same time a difficulty in cutting and abrasion, in proportion to the degree of wear resistance, and hence have a disadvantage that coater dies can only be formed in a very high cost. They also have additional disadvantages as follows: In the case of the hard metals, the wear can not be prevented since some of the fine particles contained in coating solutions have a higher hardness than the hard metals. In the case of the ceramic materials, the die edges tend to chip because of brittleness of the materials and hence it is difficult to delicately shape the edges, in particular, to work sharp angular portions, making it impossible to obtain die edges capable of achieving ideal coatability. They give another disadvantage that, when the shape of die edges has gone beyond the bounds of required precision, the whole coater die must be disused and a new one must be prepared, which is uneconomical and cumbersome.